1. Field of the Invention
This invention relates to a process for fabricating lightweight ceramic mirrors and, more particularly, to the fabrication of lightweight silicon/silicon carbide (Si/SiC) mirrors by means of a chemical vapor deposition process.
2. Description of the Prior Art
In the field of optics, light detection and ranging (LIDAR) has come to be recognized as an important diagnostic tool for remote measurement of a diversity of atmospheric parameters such as minor species concentrations, pressure, temperature, and water vapor profiles, aerosol cloud distributions, and wind fields. LIDAR techniques such as measurement of back scattered signals, differential absorption, and Doppler shifts have been used to obtain information about the earth's atmosphere.
The performance of a LIDAR system depends upon the optical configuration of its receiving telescope. Often, due to space limitations such as in a shuttle-borne LIDAR system, the length of the telescope is fixed. Therefore, the optical designer must select a particular shape and optics speed of the mirrors to maximize the throughput of the telescope. The most critical element of the receiving telescope is the primary mirror because of its size, weight, fabrication cost, and thermal exposure to the outside world. Since the received signal is directly proportional to the area of the primary mirror, it is important to use as large a primary mirror as feasible to obtain reasonable signal levels for accurate measurement. This is particularly true when a space-borne LIDAR system is used to measure wind profiles in the troposphere on a global basis.
The conventional techniques employed in the prior for fabricating large (.gtoreq.1.0 meter diameter) mirrors are quite slow and time consuming. Several months to years are required to fabricate a large mirror from ultra low expansion silica glass or Zerodur, a product commercially available from Schott Glass Technologies, Inc., 400 York Avenue, Duryea, Penna. 18642. Since a number of space-based LIDAR systems are planned for the future, considerable attention is currently being given to the development of techniques for the rapid and economic production of large, high performance mirrors.
Thus, a spin casting technique has been proposed to fabricate 1.2 meter and 3, 5 meter diameter glass mirror blanks containing lightweight honeycomb cells. Although this technique is relatively faster than the conventional mirror fabrication methods and produces lightweight mirrors, the weight of these mirrors is still an order of magnitude more than permissible for many space applications. Further, the spin-casting technique is unsuitable for fabricating large mirrors of advanced ceramics such as SiC, titanium diboride (TiB.sub.2), and boron carbide (B.sub.4 C) that have high melting points. These latter materials have properties superior to those of glass for large lightweight optics.
Other techniques involving the casting of fiber reinforced composites containing epoxy and plastics and the stretching of membranes over appropriate substrates are also currently under investigation.
Thus, there is a need and a demand for improvement in the methods of fabrication of high performance optics, in large and small sizes, rapidly, efficiently, and in a cost effective manner. Large, lightweight optical components, in particular, mirrors, are required for many space-based systems such as LIDAR systems, astronomical telescopes, solar collectors, high energy particle, ion and laser beam concentrators and deflectors. The present invention was devised to fill the technological gap that has existed in the art in these respects.